Concurrent with the recent and anticipated future increase in the use of diesel engine powered passenger automobiles in the United States has come an increasing interest in the development of practical methods for limiting particulate and other emissions in diesel exhaust gases to levels that will avoid any adverse environmental effects. In this connection work is underway both in the area of engine modifications and add-on devices to control diesel exhaust emissions. At the same time, efforts are in process in both government and industry to learn more about diesel engine exhaust emissions and their possible environmental effects to aid in establishing rational standards for their control.